Double super-heterodyne receiver

ABSTRACT

A double superheterodyne receiver comprising a first mixer responsive to a frequency modulated signal and a first local oscillator signal for providing a first intermediate frequency signal having a frequency f 1  and a positive side frequency deviation of +Δf 1  and a negative side frequency deviation of -Δf 2  where Δf 1  =Δf 2 , a second mixer responsive to the first intermediate frequency signal and a second local oscillator signal having a frequency f s  for providing a second intermediate frequency signal having a frequency of f 2  and harmonics nf 2  and (n+1)f 2  where n is positive integer indicating the harmonic of f 2  in the lower side of f 1  which substantially deviates to f 1  +Δf 1  or to f 1  -Δf 2  and n+1 indicates the harmonic of f 2  in the upper side of f 1  which substantially deviates to f 1  -Δf 2   (when nf 2  substantially deviates to f 1  +Δf 1 ) or to f 1  +Δf 1  (when nf 2  substantially deviates to f 1  -Δf 2 ).

BACKGROUND OF THE INVENTION

This invention concerns a double super heterodyne receiver.

In general, because carrier waves are amplified by two narrow-bandamplification circuits, consisting of a radio frequency amplificationcircuit and an intermediate frequency amplification circuit, superheterodyne receivers are characterized by their ability to stabilizecarrier wave amplification to the detector and by suitable selectivity.

However, FM receivers involve the following. Carrier waves reaching theFM receiver are obstructed by noise within the passband of the selectiveamplifier up to the FM detector. That appears as so-called FM noise inthe output of the FM detector. However, this noise diminishes in inverseproportion to the electrical power of the carrier wave reaching thereceiver. In other words, it is proportional to the signal-to-noise (SN)ratio. Accordingly, since the FM noise is below the residual noise ofthe FM detector at a sufficient carrier wave level, the limit of the SNratio of the receiver demodulation output would be determined by theratio of the FM detector output to the residual noise of the detector.The residual noise of the FM detector varies with the design, andimprovement must be made in the products which constitute the FMreceiver in order to reduce this noise. The SN ratio of the FM detectoroutput can also be improved by improving the detection efficiency,however.

With this in mind, we will next investigate the properties of the FMdetector from the demodulation theory of FM waves. In FM wave detection,FM waves are imposed on a circuit in which the amplitude or phasechanges linearly in relation to the deviation in frequency and detectionis carried out by detection of the phase or amplitude deviation of theoutput signal. FM detectors using the former method include twin tuningdetectors, Forster Seeley detectors and radio detectors, while FMdetectors using the latter method include quadrature detectors and pulsecount type detectors.

We will first consider the properties of an FM detector based on thefirst method.

When the frequency modulated wave is modulated by the modulating waveS(t) expressed by the following formula (1), the frequency modulatedwave i(t) is represented by formula (2):

    S(t)=I.sub.s cos pt                                        (1)

    i(t)=I.sub.O sin (ω.sub.O t+m.sub.f sin pt).         (2)

In formula (2), however, I_(O) is the amplitude of the carrier wave,ω_(O) is the angular frequency of the carrier wave and m_(f) is themodulation index. The modulation index m_(f) is represented by thefollowing formula when the maximum angular frequency deviation is takenas Δω and the constant is K.

    m.sub.f =(Δω/p)=(KI.sub.s /p.)                 (3)

When a frequency modulated wave i(t) is imposed on a circuit whoseoutput frequency changes linearly in relation to the frequency, such asa differential circuit, the output i'(t) would be represented asfollows:

    i'(t)=I.sub.O ω.sub.O (1+m.sub.a cos pt) cos (ω.sub.O t+m.sub.f sin pt).                                                  (4)

The following is also true:

    m.sub.a =Δω/ω.sub.O.                     (5)

Specifically, if the frequency modulated wave i(t) passes through thedifferential circuit, the output will be a amplitude modulated wave withmodulation index m_(a) and envelope cos pt. Accordingly, demodulation ofthe frequency modulated wave is possible if envelope line detection ofthis output i'(t) is conducted. If we assume that the maximum angularfrequency deviation Δω is constant, then the degree of modulation m_(a)increases as the angular frequency of the carrier wave ω_(O) decreases.

Next we will consider the properties of an FM detector based on thesecond method. When the frequency modulated wave i(t), represented bythe same formula (2) as in the previous case, is imposed on a circuit inwhich the output phase deviates linearly in relation to the frequencydeviation, such as a phase shift circuit, the following formuladevelops, assuming the central phase angle of the carrier wave angularfrequency ω_(O) to be θ_(O), the deviation of the output phase angle inrelation to frequency change to be τ and the phase differential inrelation to the input of the phase shift circuit output to be θ(t).##EQU1## τ is called the delay time of the phase shift circuit.Accordingly, demodulation of the frequency modulated wave would bepossible if an electronic circuit were designed in which the voltagewere proportional to the phase difference between the output side andthe input side of the phase shift circuit expressed in formula (6). Ifthe maximum angular frequency deviation Δω were assumed to be constant,the phase difference would increase directly with the delay time τ.Thus, a large demodulation signal would be achieved. In addition, thedelay time τ would be greater in relation to the phase shift circuit ofsame structure as the angular frequency of the carrier wave ω_(O) becamesmaller. Using a pulse counter detector as an example, the pulse widthwould be greater the smaller the carrier wave frequency. This is theselection of a large delay time τ.

As indicated above, if the maximum angular frequency deviation Δω isconstant, the FM detector will have the property of an improveddetection efficiency the smaller the carrier wave angle frequency ω_(O).Accordingly, the use of the super heterodyne method in the FM receiverwould result in higher detection efficiency, improved SN ratio and theready achievement of a better SN ratio the lower the intermediatefrequency selected.

However, the intermediate frequency amplification circuit in the FMreceiver ensures the necessary occupied bandwidth in FM broadcast andreception and, since selective amplification with a sufficient bandwidthin relation to fluctuation of the oscillation frequency of the localoscillator must be conducted, the intermediate frequency cannot beeasily lowered. The double super heterodyne method was conceived toeliminate such problems. Specifically, the first intermediate frequencyis set at the frequency value so that said problems do not develop.Frequency conversion of this first intermediate frequency is repeatedand a second intermediate frequency is achieved with a still lowerfrequency value. The detection efficiency by FM detection is improved.

However, although the double super heterodyne method solves variousproblems, it does give rise to new ones. Specifically, when the firstintermediate frequency output or the second local oscillator outputleaked out in the second intermediate frequency output, the frequencyvalue of the higher harmonic of the second intermediate frequencyapproaches the values of the first intermediate frequency or of thesecond local oscillator output frequency and a beat component developswith frequency equal to the difference between the two. This beatcomponent appears in the demodulation output as noise.

SUMMARY OF THE INVENTION

This invention provides a double super heterodyne FM receiver whicheliminates disturbance due to the beat component which develops betweenthe higher harmonic frequency of said second intermediate frequency andthe first intermediate frequency or the second local oscillator output.

This invention will be described below with reference to one example inthe figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of an example of a double superheterodyne FMreceiver of this invention, while FIGS. 2 and 3 are illustrations whichdescribe the invention operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 is a block diagram of an example of a double super heterodyne FMreceiver using this invention. 1 is the antenna, 2 is the radiofrequency amplification circuit, 3 is the first local oscillationcircuit, 4 is the first mixer circuit, 5 is the first intermediatefrequency amplification circuit, 6 is the second local oscillationcircuit, 7 is the second mixer circuit, 8 is the second intermediatefrequency amplification circuit, 9 is the FM detection circuit, 10 isthe audio frequency amplification circuit, and 11 is the speaker. Thefirst local oscillation circuit 3 is a variable oscillation circuit fortuning while the second local oscillation circuit 6 is a fixedoscillation circuit.

In the circuit design according to this invention, the secondintermediate frequency f₂ is set so that the first intermediatefrequency (called f₁) resulting from the first mixer circuit 4 issituated in the dynamic center of the nth order higher harmonicfrequency nf₂ of the second intermediate frequency resulting from thesecond mixer circuit 7 (called f₂) and the N+1)st order higher harmonicfrequency (n+1)f₂. n is a positive integer.

Here we will explain the meaning of `dynamic`. In the case of the lowerside heterodyne, the following expression results when the second localoscillation frequency is taken as f_(s) :

    f.sub.2 =f.sub.1 -f.sub.s.                                 (7)

When the first intermediate frequency f₁ changes due to frequencydeviation of the received FM waves or due to detuning, the amount ofpositive side change is assumed to be +Δf₁ while the amount of negativeside change is assumed to be -Δf₂. If the nth order higher harmonicfrequency of the second intermediate frequency f₂ which is nearest inthe lower side of the first intermediate frequency f₁ were taken as nf₂and if the (n+1)st order higher harmonic frequency of the secondintermediate frequency f₂ which is nearest in the upper side were takenas (n+1)f₂, nf₂ and (n+1)f₂ would undergo changes of only +nΔf₁ and+(n+1)f₂, respectively, when the first intermediate frequency f₁ changedby +Δf₁, while the changes in nf₂ and (n+1)f₂ would be -nΔf₂ and-(n+1)f₂, respectively, if the first intermediate frequency f₁ changedby an amount -Δf₂. Accordingly, when the first intermediate frequency f₁undergoes a change of +Δf₁ the following formula will emerge if weassume the development of a beat component between it and the nth orderhigher harmonic frequency of the second intermediate frequency f₂ afterthe accompanying change of +nΔf₁ :

    f.sub.1 +Δf.sub.1 =n(f.sub.2 +αf.sub.1).       (8)

At the same time, if the first intermediate frequency f₁ undergoes achange -Δf₂, the following formula will emerge if we assume thedevelopment of a beat component between it and the (n+1)st order higherharmonic frequency of the second intermediate frequency f₂ after theaccompanying change of -(n+1)f₂ :

    f.sub.1 -Δf.sub.2 =(n+1)(f.sub.2 -Δf.sub.2).   (9)

Formula (10) emerges in both formulas (8) and (9):

    Δf.sub.1 =Δf.sub.2 (=Δf).                (10)

If the frequency values of the first intermediate frequency f₁ andsecond intermediate frequency f₂ are chosen in this manner, then whenthe first intermediate frequency f₁ deviates, the beat component betweenthe nth order higher harmonic frequency of the second intermediatefrequency f₂ and the (n+1)st order higher harmonic frequency present inboth sides of the first intermediate frequency f₁ will develop least.

Thus, the following formula will be derived, to find the secondintermediate frequency f₂ from formulas (8), (9) and (10): ##EQU2## Inaddition, formula (12) will emerge, to find the second local oscillationfrequency f₂ from formulas (7) and (11): ##EQU3##

FIG. 2 illustrates the frequency positional relation in the case of thelower heterodyne noted previously.

The following expression emerges in the case of the upper sidehetrodyne:

    f.sub.2 =f.sub.s -f.sub.1.                                 (13)

In this case, when the first intermediate frequency f₁ undergoesfrequency deviation, the second intermediate frequency f₂ undergoes afrequency deviation in the relation opposite that in the case of thelower side heterodyne. Accordingly, the following formula emerges uponthe +Δf₁ change of the first intermediate frequency and the accompanying-(n+1)Δf₁ change, whereupon the beat component develops between it andthe (n+1)st order higher harmonic frequency of the second intermediatefrequency f₂.

    f.sub.1 +Δf.sub.1 =(n+1)(f.sub.2 -Δf.sub.1).   (14)

At the same time, when the first intermediate frequency f₁ undergoes achange -Δf₂, the following formula emerges upon the development of abeat component between it and the nth order higher harmonic of thesecond intermediate frequency f₂ after the accompanying change of +nΔf₁:

    f.sub.1 -Δf.sub.2 =n(f.sub.2 +Δf.sub.2).       (15)

Thus, the following formula emerges to find the second intermediatefrequency f₂, by substituting the conditions of formula (1) intoformulas (14) and (15), so that the first intermediate frequency f₁ liesin the dynamic center of the higher harmonic frequency of the secondintermediate frequency f₂ : ##EQU4## In addition, the following formulaemerges to find the second local oscillation frequency f_(s) throughformulas (13) and (16): ##EQU5##

FIG. 3 illustrates the frequency positional relation in the case of theupper heterodyne noted previously.

As indicated above, elimination of the beat component which developsbetween the second local oscillation frequency f_(s) and the higherharmonic frequency of the second intermediate frequency f₂ can beachieved by establishment of the second intermediate frequency f₂ ; thedetails are omitted here.

This invention will next be discussed in the case of use of FM stereoreceivers currently in operation.

Since current FM stereo receivers involve f₁ =10.7 MHz, in the case oflower side heterodynes, n=5 when the demodulation bandwidth isconsidered and the peak conversion ratio is sought for the SN ratio.Accordingly, when f₁ =10.7 MHz and n=5 are substituted into formula(11), f₂ =1.965 MHz develops.

As noted above, using this invention, since the second intermediatefrequency in the double super heterodyne FM receiver is designed so thatthe first intermediate frequency will be situated in the dynamic centerbetween the nth order higher harmonic of the second intermediatefrequency and the (n+1)st order higher harmonic frequency, thedisturbance due to the beat component which develops between the higherharmonic frequency of the second intermediate frequency and the firstintermediate frequency or the second local oscillation output can beminimized. This simplifies the design of the second intermediatefrequency filter and disturbance would not even develop if the higherharmonic component of the second intermediate frequency which developsin the detector, etc. were to rush into the first intermediate frequencyamplification state due to radiation.

Thus, if the double super heterodyne method of this invention is appliedwith establishment of the second intermediate frequency and combinationwith a linear detector in that frequency band, such as a pulse counterdetector, a fine FM receiver would result without beat disturbance, withimproved detection efficiency and an improved SN ratio.

I claim:
 1. A method of double superheterodyning a frequency modulatedsignal comprising the steps ofmixing the frequency modulated signal witha first local oscillator signal to provide a first intermediatefrequency signal having a frequency f₁ and a positive side frequencydeviation of +Δf₁ and a negative side frequency deviation of -Δf₂ whereΔf₁ =Δf₂, and then mixing the first intermediate frequency signal with asecond local oscillator signal having a frequency f_(s) less than f₁ toprovide a second intermediate frequency signal having a frequency of f₂and harmonics nf₂ and (n+1)f₂ where the relationship between f₁ and f₂is such that n is a positive integer indicating the harmonic of f₂, nf₂,in the lower side of f₁ which substantially deviates to f₁ +Δf₁ and n+1indicates the harmonic of f₂, (n+1)f₂, in the upper side of f₁ whichsubstantially deviates to f₁ -Δf₂.
 2. A method as in claim 1 where f₁=10.7 MHz.
 3. A method as in claim 1 where Δf₁ =Δf₂ =Δf and where 2Δfcorresponds to the demodulation bandwidth.
 4. A method as in claim 3where f₁ =10.7 MHz.
 5. A method as in claim 4 where f₂ substantially is1.965 MHz.
 6. A method of double superheterodyning a frequency modulatedsignal comprising the steps ofmixing the frequency modulated signal witha first local oscillator signal to provide a first intermediatefrequency signal having a frequency f₁ and a positive side frequencydeviation of +Δf₁ and a negative side frequency deviation of -Δf₂ whereΔf₁ =Δf₂, and then mixing the first intermediate frequency signal with asecond local oscillator signal having a frequency f_(s) greater than f₁to provide a second intermediate frequency signal having a frequency off₂ and harmonics nf₂ and (n+1)f₂ where the relationship between f₁ andf₂ is such that n is a positive integer indicating the harmonic of f₂,nf₂, in the lower side of f₁ which substantially deviates to f₁ -Δf₁ andn+1 indicates the harmonic of f₂, (n+1)f₂, in the upper side of f₁ whichsubstantially deviates to f₁ +Δf₂.
 7. A method as in claim 6 wherein f₁=10.7 MHz.
 8. A method as in claim 6 where Δf₁ =Δf₂ =Δf and where 2Δfcorresponds to the demodulation bandwidth.
 9. A method as in claim 8where f₁ =10.7 MHz.